Effects of microstructure characteristics on the tensile properties and fracture toughness of TA15 alloy fabricated by hot isostatic pressing

Powder hot isostatic pressing(HIP) is an effective method to achieve near-net-shape manufacturing of high-quality complex thinwalled titanium alloy parts, and it has received extensive attention in recent years. However, there are few reports about the microstructure characteristics on the strengthening and toughening mechanisms of powder hot isostatic pressed(HIPed) titanium alloys. Therefore, TA15powder was prepared into alloy by HIP approach, which was used to explore the microstructure characteristics at different HIP temperatures and the corresponding tensile properties and fracture toughness. Results show that the fabricated alloy has a “basket-like structure” when the HIP temperature is below 950℃, consisting of lath clusters and surrounding small equiaxed grains belts. When the HIP temperature is higher than 950℃, the microstructure gradually transforms into the Widmanstatten structure, accompanied by a significant increase in grain size. The tensile strength and elongation are reduced from 948 MPa and 17.3% for the 910℃ specimen to 861 MPa and 10% for the 970℃ specimen.The corresponding tensile fracture mode changes from transcrystalline plastic fracture to mixed fracture including intercrystalline cleavage.The fracture toughness of the specimens increases from 82.64 MPa·m^(1/2)for the 910℃ specimen to 140.18 MPa·m^(1/2)for the 970℃ specimen.Specimens below 950℃ tend to form holes due to the prior particle boundaries(PPBs), which is not conducive to toughening. Specimens above 950℃ have high fracture toughness due to the crack deflection, crack branching, and shear plastic deformation of the Widmanstatten structure. This study provides a valid reference for the development of powder HIPed titanium alloy.

Mechanical properties and microstructure of an α+β titanium alloy with high strength and fracture toughness

The Ti-Al-Sn-Zr-Cr-Mo-V-Si （Ti-62A） alloy, an alpha-beta alloy with high strength and fracture toughness, is currently used as an advanced structural material in aerospace and non-aerospace applications. Thermo-mechanical processes can be used to optimize the relationship between its strength and fracture toughness. A Ti-62A alloy bar can be machined through a transus β-forged plus α＋β solution treated and aged specimen with a lamellar alpha microstructure. The effects of heat treatment on the mechanical properties were discussed. Heat treatment provided a practical balance of strength, fracture toughness, and fatigue crack growth resistance. A comparison of the Ti-62A alloy with the Ti-62222S alloy under the same thermo-mechanical processing conditions showed that their properties are at the same level.

Microstructure-fracture toughness relationships and toughening mechanism of TC21 titanium alloy with lamellar microstructure

The independent influence of microstructural features on fracture toughness of TC21alloy with lamellar microstructure was investigated.Triple heat treatments were designed to obtain lamellar microstructures with different parameters,which were characterized by OM and SEM.The size and content ofαplates were mainly determined by cooling rate from singleβphase field and solution temperature in two-phase field;while the precipitation behavior of secondaryαplatelets was dominantly controlled by aging temperature in two-phase field.The content and thickness ofαplates and the thickness of secondaryαplatelets were important microstructural features influencing the fracture toughness.Both increasing the content ofαplates and thickeningαplates(or secondaryαplatelets)could enhance the fracture toughness of TC21alloy.Based on energy consumption by the plastic zone of crack tip inαplates,a toughening mechanism for titanium alloys was proposed.

Effect of titanium addition on fracture toughness behavior of ZL108 alloy

Two different titanium alloying methods were applied to ZL108 alloy for preparing specimens containing titanium. The specimens were tested on the MTS 810 material test system for studying their behavior of the plane strain fracture toughness KIC.The experimental data were analyzed by the statistical significance tests. The results show that the fracture toughness of the ZL108 alloy containing titanium is superior to that of common ZL108 alloy containing no titanium, but there is no significant difference for different titanium alloying methods. Therefore titanium addition is an effective method for improving the fracture toughness of the alloy ZL108.

Dynamic fracture toughness of TA15ELI alloy studied by instrumented impact test

The dynamic fracture toughness of TA15ELI alloy with two types of microstructures was studied by instrumented impact test.Charpy specimens with both the 0.2 mm U-notch and the a/W = 0.2 pre-crack were adopted to compare notch sensitivity in the two microstructures.The result shows that the specimen with Widmanst？tten microstructure exhibits a better dynamic fracture toughness and lower notch sensitivity than that with lath-like microstructure.Fracture surfaces in the case of the two microstructures are analyzed to have a ductile and brittle mixed feature under dynamic loading.The fracture surface of lath-like microstructure is composed of dimples and tear ridges,while that of Widmanst？tten microstructure is covered with rough block-like facets and dimples and tear ridges.The α phase boundaries and α/β interfaces act as locations for void nucleation and crack arrest and deviation.The decrease in width of α phase lamellae leads to the increase in the amount of boundaries and interfaces,which causes the increase in the consumption of impact energy and results in the improvement in dynamic fracture toughness.

α″phase-assisted nucleation to obtain ultrafineαprecipitates for designing high-strength near-βtitanium alloys

The diffusion-multiple method was used to determine the composition of Ti−6Al−4V−xMo−yZr alloy(0.45<x<12,0.5<y<14,wt.%),which can obtain an ultrafine α phase.Results show that Ti−6Al−4V−5Mo−7Zr alloy can obtain an ultrafineαphase by using the α″phase assisted nucleation.The bimodal microstructure obtained with the heat-treatment process can confer the alloy with a good balance between the strength and plasticity.The deformation mechanism is the dislocation slip and the{1101}twinning in the primary α phase.The strengthening mechanism is α/β interface strengthening.The interface of(0001)α/(110)β has a platform−step structure,whereas(1120)α/(111)βinterface is flat with no steps.

Effect of microstructure on impact toughness of TC21 alloy

The impact toughness of TC21 alloy after different types of forging and heat treatments was studied. The results show that heat treatment at 915 ℃ for 1 h followed by air-cooling can achieve the highest impact toughness. The crack propagation path of bimodal microstructure is different from that of lamellar microstructure. Boundaries of primary α grain are observed to be preferential sites for microcrack nucleation. With the increase of heat treatment temperature,the volume fraction of primary α phase decreases and the nucleation sites of microcrack at the primary α phase boundaries also decrease,the impact toughness value is effectively improved. The microcracks of lamellar microstructure are located on α/β interface,or the boundary of colony,and/or grain boundary α phase. The crack propagates cross the colony,or along the colony boundary,and/or along β grain boundary. The crack propagation path of lamellar microstructure is dependent on the size,direction of colony. The crack path deflects at grain boundaries,colony boundaries,or arrests and deviates at α/β interface because of crisscross α lamellar. Therefore the impact toughness value of basket microstructure is higher than that of Widmanstatten microstructure.

Microstructure and mechanical properties of a new high-strength and high-toughness titanium alloy

In order to develop a new titanium alloy with a good combination of strength-ductility-toughness,a nearbeta titanium alloy was designed based on the already widely used Ti-1023 alloy.To avoid beta fleck occurring in the microstructure,the new Ti-Al-Fe-V(Cr,Zr) alloy has been made through decreasing the content of Fe,based on molybdenum equivalency and Bo-Md molecular orbital method(a method for new alloy designing based on the molecular orbital calculating).After primary design computation,Ti-Al-Fe-V(Cr,Zr) alloy was optimized as Ti-3Al-4.5Cr-1Fe-4V-1Zr finally.The microstructure and tensile properties of this alloy subjected to several commonly used heat treatments were investigated.The results show that the tensile strength of the alloy after solution treated below the β-transus temperature comes between 850 and 1100 MPa,with elongation in the range of 12.5 %-17.0 %.In solution-treated and solution-aged samples,a low-temperature aging at 500 ℃ results in the precipitation of finer α phase.With the increase in aging temperature,the secondary α phase becomes coarser and decreases in amount.Thus,it will lead to the increase in tensile ductility,but reduction in strength.Eventually,after modulated aging treatment,the alloy can obtain highstrength level with acceptable ductility.The tensile strength of the alloy can achieve 1273 MPa,with an elongation of 11.0 %.At the same time,the fracture toughness(K_(IC)) of the alloy achieves 83.8 MPa·m^(1/2).It is obvious that the newly designed alloy has achieved a good blend of strength-ductility-toughness.

Subsurface deformation mechanism and the interplay relationship between strength–ductility and fretting wear resistance during fretting of a high-strength titanium alloy

Fretting wear damage of high-strength titanium fasteners has caused a large number of disastrous accidents.Traditionally,it is believed that both high strength and excellent ductility can reduce fretting wear damage.However,whether strength and ductility are contradictory or not and their appropriate matching strategy under the external applied normal stress(Fw)are still confusing problems.Here,by analyzing the subsurface-microstructure deformation mechanism of several samples containing variousαprecipitate features,for the first time,we design strategies to improve fretting damage resistance under different matching relation between Fw and the tensile strength of materials(Rm).It is found that when Fw is greater than Rm or Fw is nearly equivalent to Rm,the deformation mechanism mainly manifests as serious grain fragmentation ofβandαGB constituents.Homogeneous deformation in large areas only reduces damage to a limited extent.It is crucial to improve the strength to resist cracking and wear,but it is of little significance to improve the ductility.However,when Fw is far less than Rm,coordinated deformation ability reflected by ductility plays a more important role.The deformation mechanism mainly manifests as localized deformation ofβandαGB constituents(kinking induced by twinning and spheroidizing).A unique composite structure of nano-grained/lamellar layer and localized deformation transition layer reduces fretting damage by five times compared with a single nanograined layer.Only when the strength is great enough,improving the plasticity can reduce wear.This study can provide a principle for designing fretting damage resistant alloys.

New high-strength Ti–Al–V–Mo alloy: from high-throughput composition design to mechanical properties

The high-throughput diffusion-multiple technique and thermodynamics databases were used to design new high-strength Ti alloys. The composition–microstructure–property relationships of the Ti64–xMo alloys were obtained. The phase fraction and composition of the α and β phases of the Ti64–xMo alloys were calculated using the Thermo-Calc software. After aging at 600℃, the Ti64–6 Mo alloy precipitated ultrafine α phases. This phenomenon was explained on the basis of the pseudo-spinodal mechanism by calculating the Gibbs energy curves of the α and β phases of the Ti64–xMo alloys at 600℃. Bulk forged Ti64–6 Mo alloy exhibited high strength and moderate plasticity after α/β-phase-field solution treatment plus aging. The tensile properties of the alloy were determined by the size and morphology of the primary and secondary α phases and by the β grain size.

Effect of Post Weld Heat Treatment on Microstructure and Fracture Toughness of Friction Welded Joint

The effect of post weld heat treatment on the microstructure and fracture toughness of friction welded joints of Ti-6.5Al-1Mo-1V-2Zr alloy was studied. The experimental results show that equiaxial grains were formed at the center of the weld metal while highly deformed grains were observed in the thermomechanically affected zone. The fracture toughness of the weld metal was lower than that of the thermomechanically affected zone under as-weld and post weld heat treatment conditions. With increasing temperature of post weld heat treatment, the fracture toughness of weld center and thermomechanically affected zone increased. The fractographic observation revealed that the friction welded joints fractured in a ductile mode.

BASCA热处理对TC10钛合金组织与断裂韧性的影响

本工作对TC10钛合金进行BASCA热处理(β退火+缓慢冷却+时效),通过改变BA温度(β退火温度),研究BASCA热处理对TC10钛合金微观组织与断裂韧性的影响。结果表明:BA温度对合金微观组织与断裂韧性起到决定作用,随着BA温度升高,合金微观组织类型由等轴组织转变为片层组织,断裂韧性不断增加,最大值为77 MPa·m^(1/2)。此外,研究了不同类型组织的裂纹尖端区域塑性变形量、裂纹扩展路径以及断口微观形貌,进一步揭示断裂机制。与片层组织相比,等轴组织的裂纹尖端区域塑性变形较大,裂纹扩展路径曲折程度较小。等轴组织的断口形貌光滑平顺,主要由韧窝构成;片层组织的断口形貌凹凸起伏明显,韧窝数量与尺寸均减小,并出现撕裂棱、空洞以及二次裂纹。

12 mm厚TC4钛合金激光-MIG复合焊接头组织与性能研究

目的探究TC4激光-MIG复合焊接头显微组织与基本力学性能之间的联系,分析接头不同区域的断裂行为。方法利用激光-MIG复合焊制备TC4钛合金对接接头,采用光学显微镜和扫描电镜观察接头焊缝区、热影响区及母材的显微组织,在室温下进行了显微硬度测试、拉伸性能测试与断裂韧性测试,并对试样断口进行了观察分析。结果接头的焊缝区组织为粗大的β相柱状晶,晶内纵横分布着αʹ针状马氏体和针状α相,靠近焊缝一侧的热影响区则由针状αʹ相、α集束与少量细小的块状α相构成。随着远离焊缝中心,母材侧热影响区组织转变为块状的α相、少量α集束及初生β相,并最终趋于与母材组织相似。热影响区的显微硬度值达到最大,这是因为该区域存在比焊缝区更为细小的针状αʹ相。接头的平均抗拉强度和断后伸长率分别为1020.22 MPa和7.38%。接头在拉伸时主要在焊缝区发生断裂。焊缝区展现出比母材区和热影响区更优异的断裂韧性,平均值为87.14 MPa·m1/2,焊缝区内纵横交错的网篮组织与集束是其断裂韧性较高的主要原因。结论在TC4钛合金的激光-MIG复合焊过程中,针状α相和αʹ马氏体的存在会提高焊缝的显微硬度和断裂韧性,但相较于母材塑性没有提升,通过调控焊缝区显微组织结构,可以获得所需性能的接头。

准β锻造和β热处理对TC11组织和断裂韧性的影响

研究了准β锻造和β热处理对TC11钛合金显微组织和断裂韧性的影响。结果表明TC11钛合金经准β锻造及β热处理后断裂韧性相较于魏氏组织明显提升,延伸率提升约50%,断面收缩率提升约20%,KIC提升约20%。由于准β锻造改变了层片状α相的析出形态,形成了取向差异化明显的束集,抵抗裂纹扩展的组织协调性更佳,从而使得断裂韧性更优;而魏氏组织试样主要以晶界和大片次生α相增加裂纹扩展曲折性。

TA15钛合金断口组织特征与持久断裂关联性的研究

通过一系列高温持久试验,结合金相显微镜、扫描电镜组织分析,研究了TA15钛合金持久断口附近晶粒分布以及断口附近的变形特征与持久断裂的关联性。结果表明:高温400℃、加载应力为440 MPa时,相比较于纵向试样,横向试样的持久寿命较长,说明其持久性能较优异;观察断裂位置和夹持端的组织特征后,再与持久断裂时间比较发现,断口处的晶粒尺寸明显大于夹持端的晶粒尺寸,并且持久断口处的组织形态有明显拉长;扫描电镜下的断口组织显示,试样断裂形式均为韧窝聚集型断裂,经对比发现,横向试样的裂纹扩展途径曲折,并且韧窝比较深,纵向试样韧窝呈现出不规则形状,韧窝较浅。

时效温度对TA10钛合金微观组织以及冲击韧性的影响

对TA10钛合金进行固溶时效处理,研究TA10钛合金在不同时效温度条件下微观组织形貌与冲击韧性二者之间的关系。研究发现:在固溶条件下,合金组织结构主要由α′相和初生α相组成,再经时效处理后,组织中初生α相的形貌以及体积分数均未出现明显变化,且有大量次生α相析出,并有明显的β转变组织形成。合金经固溶时效处理后的冲击韧性,较固溶处理后的数值更低,而在固溶时效条件下,随着时效温度的逐步提高,合金的冲击韧性整体呈现出降低的趋势,但整体降低趋势并不十分明显。发现无论是固溶还是固溶时效条件下,其冲击断口形貌均是由大量韧窝形貌构成,且固溶条件下,冲击断口中的韧窝数量更多,其断口整体较为平稳,同时发现断口中空洞的数量和尺寸在时效温度升高的过程中也不断增加。

Unravelling the competitive effect of microstructural features on the fracture toughness and tensile properties of near beta titanium alloys

The competitive effect of microstructural features including primaryα(α_(p)),secondaryα(α_(s)),grain boundaryα(α_(GB)) and β grain size on mechanical properties of a near β Ti alloy were studied with two heat treatment processes.The relative effect of β grain size and STA(solution treatment and ageing)processing parameters on mechanical properties were quantitatively explored by the application of Taguchi method.These results were further explained via correlating microstructure with the fracture toughness and tensile properties.It was found that large numbers of fine as precipitates and continuous α_(s) played greater roles than other features,resulting in a high strength and very low ductility(<2%)of STA process samples.The β grain size had a negative correlation with fracture toughness.In the samples prepared by BASCA( β anneal slow cooling and ageing)process,improved ductility and fracture toughness were obtained due to a lower density ofα;precipitates,a basket-weave structure and zigzag morphology of α_(GB).For this heat treatment,an increase in prior β grain size had an observable positive effect on fracture toughness.The contradictory effect of β grain size on fracture toughness found in literature was for the first time explained.It was shown that the microstructure obtained from different processes after β solution has complex effect on mechanical properties.This complexity derived from the competition between microstructure features and the overall sum of their effect on fracture toughness and tensile properties.A novel table was proposed to quasi-quantitatively unravel these competitive effects.

Toughening effects of Mo and Nb addition on impact toughness and crack resistance of titanium alloys

Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb alloys were prepared to investigate the toughening effects ofβstabilizers Mo and Nb on impact toughness and crack resistance of titanium alloys.Instrumented Charpy impact tests showed that the total impact absorbed energy of Ti-6Al-2Mo and Ti-6Al-3Nb(∼64 J)were two times higher than that of Ti-6Al(∼30 J),indicating the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.Analysis of load-displacement curves revealed the similar crack initiation energy of Ti-6Al,Ti-6Al-2Mo and Ti-6Al-3Nb(15.4 J,16.1 J and 15.0 J,respectively).However,the higher crack propagation energy of Ti-6Al-2Mo and Ti-6Al-3Nb(46.7 J and 48.3 J,respectively)were about three times higher than that of Ti-6Al(14.4 J),indicating the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Post-mortem analysis of impact samples demonstrated that the increased dislocation density and deformation twinning were mainly responsible for the stronger resistance to crack propagation in Ti-6Al-2Mo and Ti-6Al-3Nb.Due to the invisibility of dislocation activation and deformation twinning during the Charpy impact process,a mathematical model has been proposed to evaluate the effects of Al,Mo and Nb elements on dislocation mobility based on the Yu Rui-huang electron theory.Addition of Mo and Nb elements significantly improved the dislocation mobility in Ti-6Al-2Mo and Ti-6Al-3Nb compared to that in Ti-6Al alloy.Therefore,more dislocations were activated in Ti-6Al-2Mo and Ti-6Al-3Nb which supplied the larger plastic deformation under impact loading.A dislocation-based model also has been proposed to interpret the nucleation and propagation of deformation twinning under the impact loading.Dislocation pileup atα/βinterfaces provided potential sites for nucleation of deformation twinning in Ti-6Al-2Mo and Ti-6Al-3Nb.Furthermore,deformation twinning facilitated the dislocation motion inαgrains with hard orientations.The increased dislocation mobility and deformation twinning were responsible for the stronger crack resistance as well as the higher impact toughness of Ti-6Al-2Mo and Ti-6Al-3Nb alloys.

Study on dynamic fracture behavior of TA15ELI alloy under mode-Ⅱ loading by caustics method

The dynamic fracture behavior of TA 15ELI alloy with lath-like microstructure was studied by caustics method. Specimens with double-side pre-notch were tested under the plane-stress condition at mode-II loading with a drop hammer system. Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor. The dynamic fracture toughness and crack growth velocity of TA15ELI with lath-like microstructure were determined to be 279 MPa.m1/2 and 32.6 m/s, respectively. SEM fractograph analysis showed a mixed feature of mainly plastic mode for TA 15ELI alloy in dynamic mode-II fracture. Shear localization was observed in the vicinity of the crack initiation area.

TC4钛合金EBW和TIG焊接头断裂韧性对比分析

采用电子束焊和氩弧焊方法制备的TC4钛合金接头,在4种温度条件下,测试了其焊缝、热影响区及母材的断裂韧性,并结合断口形貌、硬度分布及微观组织对比分析了断裂韧性测试结果.结果表明,TC4钛合金TIG焊接头焊缝及热影响区的断裂韧性优于钛合金母材,CTOD值随温度下降而降低;相较之下,EBW焊接头焊缝断裂韧性值较母材更低,热影响区断裂韧性与母材较接近,温度变化对其CTOD值无显著影响.钛合金TIG焊接头焊缝区域具有较低硬度值,其网篮状α相和较低比例的马氏体分布是其断裂韧性较高的根本原因;而钛合金EBW焊接头焊缝中针状马氏体的分布导致其局部硬度较高,并降低了断裂韧性.